[NEW DELHI] South Asia could be the source of a fast-spreading antibiotic-resistant strain of the bacteria Shigella sonnei that causes shigellosis, the diarrhoeal disease, says a new study.

Researchers at the Hospital for Tropical Diseases, Ho Chi Minh City, Vietnam, used next-generation sequencing technology to investigate S. sonnei samples resistant to antibiotics obtained from clinics around South Asia. For comparison they also sequenced resistant samples from Australia and Ireland.

The study, results of which were published in the journal PLOS Medicine in August, showed that all of the resistant S. sonnei formed a single clade found in South Asia and that most of the positive results reported outside of Asia could be linked to recent travel to India.

“We were running a project on some shigella isolates from Bhutan that were fluoroquinolone-resistant,” corresponding author Stephen Baker, a microbiologist based at the hospital, one of Oxford University’s clinical research units, told SciDev.Net.

Fluoroquinolones are a family of broad-spectrum antibiotics, and ciprofloxacin, a fluoroquinolone, is commonly prescribed to treat shigellosis.

“We started working on some resistant strains from Vietnam also and found they were much more closely related than we thought they would be,” said Baker.

Baker and colleagues sequenced 12 resistant samples from Bhutan, 11 samples from Vietnam, and a sample each from Thailand and Cambodia. They also worked with partners to sequence 19 samples in Australia and 16 in Ireland.

“Once we had found the relationship between the Bhutanese isolates and the Vietnamese [samples], I contacted some of my collaborators and asked what strains/data they had — they all had a common ancestor,” Baker said.

Of the 45 samples obtained from outside of South Asia, 27 were from people who had travelled to the region and 80 per cent had visited India. Baker said the resistant S. sonnei strain is most likely “very common in India, but infrequently reported”.

Based on the data, Baker and his colleagues were able to pinpoint South Asia as the primary source of antibiotic-resistant S. sonnei that is spreading throughout Asia, Australia, the US, and Europe. “The genetics shows these organisms have arisen and are spreading from South Asia,” he said.

Baker attributed the outbreak to increased prosperity in South Asia and the overuse of fluoroquinolones, creating “huge selective pressure” on S. sonnei.

Jason Andrews, assistant professor of infectious diseases at Stanford University in California, said that the results of the study were both “compelling and alarming” and “underscore the urgent need to address drug-resistant pathogens through vaccination or other measures”.

In particular, Andrews noted, the antimicrobial resistance problem in Asia has quickly become a global problem, “requiring a global response in terms of resources and strategies for containing resistant enteric bacteria”.